1. Field of the Invention
This invention relates to a control system for a vehicle automatic transmission, more particularly to a control system for the lockup clutch of a torque converter of a vehicle automatic transmission.
2. Description of the Prior Art
Automatic transmissions are usually designed to automatically select the optimum gear by detecting vehicle or engine speed and the degree of throttle opening or some other such parameter indicating engine load and then retrieving a gear by the detected values of these parameters from gear shift characteristics (a shift map) determined in advance based on the same parameters.
In the automatic transmission of this type, a hydraulic torque converter equipped with a lockup clutch is installed between the internal combustion engine (power source) and the transmission unit. The torque increase characteristic of the torque converter is utilized when the vehicle is accelerating, as during drive-away or overtaking, while the lockup clutch completely or partially engages the input and output sides of the hydraulic torque converter during cruising. The ON (operative) and OFF (inoperative) regions of the lockup clutch are defined in advance in terms of the gear (gear ratio) and other driving conditions and the lockup clutch is controlled based thereon to prevent transmission efficiency loss.
Because a type of surging known as body vibration may occur in the acceleration region when the operator steps down on the accelerator pedal, drivability considerations make it extremely difficult to optimize the ON/OFF characteristics of the lockup clutch. The prior art practice has therefore been to keep the lockup clutch normally OFF when in the acceleration region is entered owing to depression of the accelerator pedal or, more precisely, to define this region as a weak lockup region in which only a weak engagement determined from the characteristics of the hydraulic circuit is imparted. A description of this prior art technology can be found, for example, in Japanese Laid-Open Patent Application No. Sho 63(1988)-180,757.
The need to expand the ON (operative) region of the lockup clutch has heightened in recent years, however, owing to increased demand for lesser fuel consumption. The prior art cannot sufficiently respond to this requirement. On the other hand, the ON (operative) region cannot be indiscriminately broadened because, as just pointed out, this would lead to surging and degrade drivability.
An object of this invention is therefore to overcome the aforesaid problems by providing a control system for a vehicle automatic transmission which expands the ON (operative) region of the lockup clutch and achieves improved fuel efficiency while avoiding the occurrence of surging.